From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
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To: | Robert Haas <robertmhaas(at)gmail(dot)com> |
Cc: | Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>, pgsql-hackers(at)postgresql(dot)org |
Subject: | Re: crash-safe visibility map, take three |
Date: | 2010-11-30 17:10:49 |
Message-ID: | 28581.1291137049@sss.pgh.pa.us |
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Lists: | pgsql-hackers |
Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> We're not going to double the cost of VACUUM to get index-only scans.
> And that's exactly what will happen if you do full-page writes of
> every heap page to set a single bit.
It's ridiculous to claim that that "doubles the cost of VACUUM". In the
worst case, it will add 25% to the cost of setting an all-visible bit on
a page where there is no other work to do. (You already are writing out
the heap page and the VM page, plus a WAL image of the heap page, so a
WAL image of the VM page adds 25%. But only if you did not set any
other bits on the same VM page, which is probably not a real common
case.) Given that VACUUM has a lot of other cleanup besides visibility
bit setting, I'm not convinced that this would even be noticeable.
I think the burden is on people who are proposing complicated mechanisms
to show that there's actually a strong need for them.
regards, tom lane
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